Premium thick Aluminium plate box, 10cm square. The vertical faces of the box are blackened, roughened, varnished and polished respectively. The top opening is fitted with a stopper. The apparatus is filled with water maintained at boiling point by a low bunsen flame. Infrared sensors may be used to compare the relative radiant heat output from each of the four faces. Stopper is supplied for the 16mm dia. top opening (stopper size 13mm bottom dia. 19mm top dia).